COMPARATIVE GEOTHERMOMETRY OF RECENT HAWAIIAN ERUPTIONS

In this paper we compare lava temperatures measured using Cr-Al thermocouples or infrared spectrometry with estimated quenching temperatures based on the glass geothermometry calibration of Helz and Thornber (1987). Comparative data are available for the April 1982 and September 1982 summit eruptions, the Pu'u 'O'o east rift eruption (1983-1986), all three eruptions at Kilauea, and the 1984 Mauna Loa eruption. The results show that quenching temperatures, based on the MgO contents of Kilauean glasses (T-MgO), lie within +/- 10 degrees C of field measurements using the infrared spectrometer for 85% of the samples. Where a Cr-Al thermocouple was used, 90% of the field measurements lie within +1 degrees to -11 degrees C of T-MgO for samples with T-field > 1130 degrees C. Samples where T-field < 1130 degrees C show larger divergence. The uncertainty in T-MgO by itself is +/- 10 degrees C, so the level of agreement between held measurements and T-MgO is very good for Kilauean lavas. Systematic comparison of field measurements df temperature with glass geothermometry for the 1984 Mauna Loa eruption suggests that, although the field and glass temperatures lie within +/- 10 degrees C of each other, the Kilauean T-MgO calibration is nevertheless not appropriate for Mauna Loa glasses and that actual quenching temperatures for Mauna Loa samples will lie 10 degrees-20 degrees C higher than would be predicted from the Kilauea calibration curve. Consideration of possible effects of variable volatile content suggest that in most cases these are small. Samples erupted early in an eruption may reflect preeruptive water contents different enough to affect T-MgO significantly, but later spatter samples and all flow samples appear to have equilibrated at low enough water contents for the calibration to be applicable. We conclude that the MgO-based geothermometer can be applied to glassy Kilauean samples to give temperatures that generally will lie within +/- 10 degrees C of a field measurement. Plots of glass MgO content versus time, if a suitable sample base is available, should give a thorough, quantitative record of the thermal history of any Kilauean eruption.